Testing with the leader, as mentioned above, is a good idea.(posted 8684 days ago)The generally accepted rule of thumb is this: you should fix for double the time it takes for the film to clear. When the time to fix the leader becomes double what it is with fresh fixer, you need new fixer.
For example ... if you need a 2:15 fix to clear the leader, use 4:30 for the actual roll of film, but when your future leaders of the same type need 4:30 to clear, your fixer is done and you should discard it.